The content of this setting is loaded as a [url:https://www.formdev.com/flatlaf/properties-files/ FlatLaf Properties File]. Properties are loaded in the following order (later loaded overwrites previous):

1. A base theme is loaded based on the selected Look&Feel (can be manually set with the property @baseTheme = <theme>, with value light, dark, intellij or darcula)
2. Chatty specific changes are added (can be disabled by adding chattyProperties = false)
3. This setting is loaded

If you just want to change the colors a bit, starting with something like this could be a good idea:

@foreground = #ddd
@background = #3d3d3d